A laser diode bar is a monolithic array of semiconductor diode lasers. All lasers in the array emit from the same facet of a cleaved die. Light is typically emitted in a direction perpendicular to the emitting facet. The output power is typically 20-150 W per bar for commercial lasers, and each bar may have an emitting length of 10 mm. Where diodes in the array are multimode lasers, the modes are highly confined in the out-of-plane direction (the “fast axis”) and have near-identical near-field distributions in this direction. The main mode structure is field variation in the in-plane direction (the “slow axis”). In the far field, these beams are highly divergent in the fast axis, and diverge more slowly in the slow axis. Fast-axis divergence may be reduced using a fast-axis collimation lens. In its simplest form this can be a cylindrical rod lens or plano-cylindrical lens placed immediately next to the emitting facet of the diode bar, although better collimation can be achieved using acylindrical lens surfaces.
Due to the high divergence of the beam and short focal length of the lens, small errors in the distance between the diode facets and the collimation lens can lead to defocusing and angular errors in the collimated beam. Ideally the bar is perfectly straight, so that the centres of all emitters lie in a straight line, but in practice, due to inherent strains and mechanical effects due to mounting, the bar is curved or “bowed” and the emitters do not lie on a straight line. Bowing means that a rod lens perfectly aligned for the first and last diodes will be misaligned for central diodes, leading to a spread in the far field patterns, and hence increased beam divergence for the overall collimated bar. Similarly, twisting or other deformation of the lens can result in misalignment giving angular error and increased divergence. Even “acylindrical” fast-axis collimating lens lenses may introduce significant aberrations, further increasing divergence.
Achieving correct collimation all the way along a 10 mm bar is extremely difficult. The effect of lens misalignments and aberrations is that the far-field beams from individual diodes point in slightly different directions and have differing angular spreads, typically exceeding the angular spread for perfect fast-axis collimation. This leads to an uneven power distribution in the far field, which results in uncontrolled and unpredictable beam spread and a reduction of the average radiance of the laser source. The commonly used plano-acylindrical rod lenses that are perfectly corrected at the centre of the angular spread of emission in the plane of the slow axis have a different focal length and significant aberrations for rays that are off-axis in this direction. This further reduces the average radiance of the laser source. It is common practice to quantify the degradation in overall beam of a laser source by measuring the M2 parameter. However, although this quantifies the overall error, a measurement of M2 does not identify the precise origins of the error.
Bars are often stacked together as a high power diode laser stack to increase the total power available from a compact source. A bar that is to be assembled into a stack may be fitted with a fast-axis collimating lens before stacking. More usually, uncollimated bars are assembled into a stack before collimating lenses are attached. Stacks are usually assembled so that beams from the constituent bars are either parallel or arranged to coincide on a specific target plane. Combining bars to form stacks therefore introduces an additional alignment requirement: the relative orientation of the beams from separate bars. Where bars have been collimated before stacking, mechanical errors in stacking lead to slight errors in the pointing direction of different bars, increasing the beam divergence of the overall beam and decreasing beam quality. More generally, mechanical misalignment between the fast-axis collimating lens and the bar leads to pointing errors in individual bars, even when there is no mechanical misalignment due to stacking.
Bars typically dissipate large amounts of heat in a small volume. Laser operation depends on temperature, and so cooling is often critical. Large temperature changes combine with differences in thermal expansion coefficient between bar and cooler, leading to a temperature-dependence of the previously described bowing effect. Thermal gradients also lead to mechanical distortions. Excessive temperatures degrade the optical performance of the bars, so water-cooling is often used to remove heat. In the case of stacks, water-cooling plates are often interleaved between the bars. Typically these are very thin and use very narrow arrays of water channels, meaning that high water pressure is needed. This combined with the very thin structure can lead to mechanical distortion, and so to further optical distortion.
Brightness and beam uniformity are key performance parameters of bars and stacks. A typical objective in the design and manufacture of collimated stacks is to achieve a uniform and intense distribution of power over some target area, with an intensity as close as possible to the theoretical limit defined by the beam quality of the individual emitters. Far-field intensity measurements can be used to quantify the overall performance of the collimated bar or stack, but do not in general identify the origins of reduced brightness, since the far-field beams cannot be easily related to the field close to the fast-axis collimation lens, particularly since the far-field power distributions that are measured typically have overlapping of beams from multiple elements in a bar and multiple bars in a stack. This is a problem, because without knowledge of the origins of the reduced brightness and/or beam uniformity, it is impossible to correct for these.
Instruments that can measure phase and power distribution over a wavefront are known, for example the Shack-Hartmann sensor. These have typically been designed to measure wavefront errors on low-power-density signals. Using them for direct measurement of high-power beams close to the laser output poses serious problems in disposing of heat if an absorbing attenuator is used. Relay optics incorporating reflective attenuators may be used to relay a low-power image of the region of interest onto the sensor. However, these can introduce unwanted aberrations and other errors, particularly in the case high power diode laser bars and stacks, whose beams have detailed transverse structure and significant divergence.